Organometallics 2006, 25, 2715-2718
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Synthesis of 2,5-Bis(binaphthyl)phospholes and Phosphametallocene Derivatives and Their Application in Palladium-Catalyzed Asymmetric Hydrosilylation Masamichi Ogasawara,*,† Azumi Ito,‡ Kazuhiro Yoshida,‡,§ and Tamio Hayashi*,‡ Catalysis Research Center and Graduate School of Pharmaceutical Sciences, Hokkaido UniVersity, Kita-ku, Sapporo 001-0021, Japan, and Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo, Kyoto 606-8502, Japan ReceiVed February 13, 2006 Summary: NoVel chiral and enantiomerically pure phospholes, which haVe axially chiral biaryl substituents at the 2- and 5-positions of the phosphole rings, haVe been prepared and conVerted into a Variety of phosphametallocenes with retention of the chiral substituents. The phospholes and the phosphametallocenes haVe been applied to palladium-catalyzed asymmetric hydrosilylation of 5,5-dimethyl-1-hexen-3-yne to giVe an axially chiral allenylsilane with high enantioselectiVity of up to 92% ee. Recently, we reported enantiomerically pure chiral phosphole 1. The compound 1 was the first example of phospholes having chiral substituents on the carbon atoms of the phosphole ring1a and could be transformed into the corresponding phospholyl anion with retention of the chiral substituents, (-)-menthyl groups. Thus, 1 could be an excellent precursor to a variety of novel chiral phosphametallocene derivatives.1 Among the phosphametallocene derivatives, phosphinophosphaferrocene 2 was applied to palladium-catalyzed asymmetric allylic alkylation as a chiral ligand and showed excellent enantioselectivity of up to 99% ee.1b Another strategy for preparing chiral phosphametallocenes is the use of unsymmetrically substituted η5-phospholides, which induces planar chirality in phosphametallocenes. Indeed, several enantiomerically pure planar chiral phosphaferrocenes were reported.2-4 Some of them were applied to transition-metalcatalyzed asymmetric reactions and displayed high potential as chiral ligands.3,4 Because these planar chiral phosphaferrocenes * To whom correspondence should be addressed. E-mail: ogasawar@ cat.hokudai.ac.jp;
[email protected]. † Hokkaido University. ‡ Kyoto University. § Present address: Department of Chemistry, Faculty of Science, Chiba University, Chiba 263-8522, Japan. (1) (a) Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics 2001, 20, 1014. (b) Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics 2001, 20, 3917. (c) Ogasawara, M.; Nagano, T.; Yoshida, K.; Hayashi, T. Organometallics 2002, 21, 3062. (d) Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics 2003, 22, 1783. (2) (a) Ganter, C.; Brassat, L.; Ganter, B. Tetrahedron: Asymmetry 1997, 8, 2607. (b) Qiao, S.; Hoic, D. A.; Fu, G. C. Organometallics 1998, 17, 773. (c) Klys, A.; Zakrzewski, J.; Rybarczyk-Pirek, A.; Olszak, T. A. Tetrahedron: Asymmetry 2001, 12, 533. (d) Agustsson, S. O.; Hu, C.; Englert, U.; Marx, T.; Wesemann, L.; Ganter, C. Organometallics 2002, 21, 2993. (3) (a) Qiao, S.; Fu, G. C. J. Org. Chem. 1998, 63, 4168. (b) Shintani, R.; Lo, M. M.-C.; Fu, G. C. Org. Lett. 2000, 2, 3695. (c) Tanaka, K.; Qiao, S.; Tobisu, M.; Lo, M. M.-C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 9870. (d) Tanaka, K.; Fu, G. C. J. Org. Chem. 2001, 66, 8177. (e) Shintani, R.; Fu, G. C. Org. Lett. 2002, 4, 3699. (f) Shintani, R.; Fu, G. C. Angew. Chem., Int. Ed. 2003, 42, 4082. (g) Shintani, R.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 19778. (h) Sua´rez, A.; Downey, W.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 11244. (4) (a) Ganter, C.; Glinsbo¨ckel, C.; Ganter, B. Eur. J. Inorg. Chem. 1998, 1163. (b) Ganter, C.; Kaulen, C.; Englert, U. Organometallics 1999, 18, 5444.
were obtained as racemates or diastereomeric mixtures, appropriate optical resolution was inevitable prior to their application in the asymmetric reactions. On the other hand, the chiral phosphametallocenes derived from our chiral phosphole 1 were obtained in enantiomerically pure form without optical resolution.1
Figure 1. Chiral phosphole 1 and phosphinophosphaferrocene 2.
Here we report the preparation of additional examples of chiral phospholes of which the design concept is analogous to that of 1. The phospholes 3a and 3b possess axially chiral atropisomeric biaryl substituents at the 2- and 5-carbons of the phosphole rings, and the latter was converted to a variety of phosphametallocene derivatives in enantiomerically pure forms. Application of these chiral compounds in Pd-catalyzed asymmetric hydrosilylation of 5,5-dimethyl-1-hexen-3-yne5a was examined, and the phosphametallocenes showed excellent enantioselectivity. Preparation of 3 is illustrated in Scheme 1. Enantiomerically pure (R)-carboxylic acid 7a, which was derived from commercially available (R)-binaphthol (4a) in three steps via binaphthyl triflate 5a and methyl ester 6a, was treated with 2 equiv of MeLi in THF at 0 °C, followed by aqueous workup, to give methyl ketone 8a in 80% yield.6 The methyl ketone 8a was converted to the corresponding terminal alkyne 9a in 90% yield according to Negishi’s protocol.7 Copper-catalyzed oxidative homocoupling of 9a afforded conjugate diyne 10a in 98% yield.8 The diyne 10a was reacted with excess PhPHLi, which was generated in situ from PhPH2 and nBuLi, to give the chiral phosphole 3a in 46% yield as a bright yellow crystalline solid.9 Preparation of the chiral phosphole 3b, which has 2-(5,5′,6,6′,7,7′,8,8′-H8-2′-MeO-binaphthyl)yl substituents, was achieved by the analogous multistep route from (R)-H8binaphthol (4b). (5) (a) Han, J. W.; Tokunaga, N.; Hayashi, T. J. Am. Chem. Soc. 2001, 123, 12915. (b) Han, J. W.; Tokunaga, N.; Hayashi, T. HelV. Chim. Acta 2002, 85, 3848. (6) Jorgenson, M. J. Org. React. 1970, 18, 2. (7) (a) Negishi, E.; King, A. O.; Klima, W. L.; Patterson, W.; Silveira, A., Jr. J. Org. Chem. 1980, 45, 2526. (b) Negishi, E.; King, A. O.; Tour, J. M. Organic Syntheses; Wiley: New York, 1990; Collect. Vol. VII, p 63. (8) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632. (9) Ma¨rkl, G.; Potthast, R. Angew. Chem. 1967, 79, 58.
10.1021/om060138b CCC: $33.50 © 2006 American Chemical Society Publication on Web 04/25/2006
2716 Organometallics, Vol. 25, No. 11, 2006
Communications
Scheme 1. Preparation of 2,5-Bis(binaphthyl)phospholes 3a and 3b
Preparation of 9a in a shorter sequence by utilizing Pdcatalyzed Sonogashira coupling was unsuccessful. Reactions of the triflate 5a with trimethylsilyl- or triphenylsilylacetylene were examined under various conditions in the presence of the Pd(PPh3)4 catalyst; however, no alkynylated products were obtained in any cases. Crystals of 3a were grown from dichloromethane/ethanol, and the solid-state structure was clarified by X-ray crystallography (Figure 2). The crystal structure of the phosphole was asymmetric with the envelope-like bent phosphole ring; the dihedral angle between the C(1)-P(1)-C(4) plane and the C(1)-C(2)C(3)-C(4) plane is 6.15°. The CR-Cβ distance is 1.35 Å (mean value), and the Cβ-Cβ distance is 1.47 Å. The difference between the two values, 0.12 Å, is quite large compared to those of the other structurally characterized phospholes,1a,10 indicating weaker electronic delocalization (less aromatic) in 3a. In accordance with these observations, the phosphorus atom in 3a shows clear pyramidal geometry (more sp3-like): the sum of the angles at the phosphorus11 is only 304°. In a phosphole with
Figure 2. ORTEP drawing of 3a with 30% thermal ellipsoids. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): P(1)-C(1) ) 1.83(1), P(1)-C(4) ) 1.85(1), C(1)-C(2) ) 1.35(2), C(2)-C(3) ) 1.47(2), C(3)-C(4) ) 1.36(2); C(1)-P(1)-C(4) ) 91.6(6), P(1)-C(1)-C(2) ) 107.7(1), C(1)-C(2)-C(3) ) 117.4(1), C(2)-C(3)-C(4) ) 113.3(1), P(1)-C(4)-C(3) ) 109.5(9), C(1)-P(1)-C(5) ) 106.1(5), C(4)P(1)-C(5) ) 106.3(5).
a bulky aryl substituent (-C6H2-2,4,6-tBu3) at the phosphorus, which shows strong aromatic character, the sum of the angles is as large as 331.7°.12 Conversion of the chiral phospholes 3a and 3b into the corresponding phospholide anions was examined. Unfortunately, however, the phosphole 3a was found to be robust to reductive cleavage of the P-Ph bond by a reaction with lithium metal or sodium naphthalenide.13 Treatment of a THF solution of 3a with lithium metal afforded an NMR-inactive dark green solution, which indicated formation of a phosphole anion radical by a single-electron transfer from the reductant.14 The anion radical showed thermal stability and did not decompose into the corresponding phospholide anion. The anion radical was generated in dioxane or xylene in a similar manner and heated to reflux for several hours. Formation of the corresponding phospholide anion, however, was not detected under these conditions. The unusual thermal stability of the anion radical could be ascribed to delocalization of anionic charge over the binaphthyl substituents. Since R,R′-diarylphospholides, such as 2,5-Ph2-phospholide or 2,3,4,5-Ph4-phospholide,13,15 could be prepared from the corresponding P-Ph phospholes by the reactions with alkaline metals, it was expected that breaking the extended conjugation of the binaphthyl substituents in 3a might reduce the thermal stability of the anion radical. As expected, the phosphole 3b, which has partially hydrogenated binaphthyl (H8-binaphthyl) groups, was cleanly converted to sodium phospholide 11 (31P NMR analysis in THF: δ +92)16 by treatment with sodium naphthalenide. It should be mentioned that generation of an (10) (a) Coggon, P.; Engel, J. F.; McPhail, A. T.; Quin, L. D. J. Am. Chem. Soc. 1970, 92, 5773. (b) Ozbirn, W. P.; Jacobson, R. A.; Clardy, J. C. Chem. Commun. 1971, 1062. (11) Jochem, G.; No¨th, H.; Schmidpeter, A. Chem. Ber. 1996, 129, 1083. (12) Keglevich, G.; Bo¨cskei, Z.; Keseru¨, G. M.; Ujsza´szy, K.; Quin, L. D. J. Am. Chem. Soc. 1997, 119, 5095. (13) Braye, E. H.; Caplier, I.; Saussez, R. Tetrahedron 1971, 27, 5523. (14) (a) Thomson, C.; Kilcast, D. Angew. Chem., Int. Ed. Engl. 1970, 9, 310. (b) Kilcast, D.; Thomson, C. Tetrahedron 1971, 27, 5705. (c) Thomson, C.; Kilcast, D. J. Chem. Soc., Chem. Commun. 1971, 214. (15) Charrier, C.; Bonnard, H.; de Lauzon, G.; Mathey, F. J. Am. Chem. Soc. 1983, 105, 6871. (16) Quin, L. D.; Orton, W. L. J. Chem. Soc., Chem. Commun. 1979, 401.
Communications
Organometallics, Vol. 25, No. 11, 2006 2717 Table 1. Pd-Catalyzed Asymmetric Hydrosilylation of 5,5-Dimethyl-1-hexen-3-yne (17) with Trichlorosilanea
Scheme 2. Generation of Phospholide 11 and Preparation of Chiral Phosphametallocenes 12-14
R-naphthyl phospholide was achieved recently by a [1,5]sigmatropic aryl shift in a P-naphthylphosphole.17 Whereas the chiral H8-binaphthyl substituents were retained in 11, the phospholide 11 served as a versatile precursor to a variety of chiral phosphametallocenes. Monophosphaferrocenes 1218 and 1319 were prepared in 78% and 64% yields, respectively, as bright red crystalline solids. Monophospharuthenocene 1420 was obtained in 39% yield (Scheme 2). The solid-state structure of the monophosphaferrocene 12 was determined by an X-ray diffraction study. As shown in Figure 3, two sterically demanding H8-binaphthyl groups construct a unique chiral pocket around the phosphorus atom. The two 2-MeO-H4-naphthyl groups occupy the side opposite the FeCp moiety with respect to the phospholyl plane, avoiding steric interaction with the Cp ligand. The phospholyl ligand and the Cp ligand are nearly parallel, with a dihedral angle of 3.07°. The Fe-Cp (1.644 Å) and Fe-phospholyl (1.648 Å) distances are nearly the same. The potential of the newly prepared compounds as chiral ligands in asymmetric synthesis was explored for the palladiumcatalyzed enantioselective hydrosilylation of 5,5-dimethyl-1hexen-3-yne (17) with trichlorosilane, forming the axially chiral allenylsilane 18,5a and the results are summarized in Table 1. In the precedent report,5a the highest enantioselectivity of 90%
Figure 3. ORTEP drawing of 12 with 30% thermal ellipsoids. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): P(1)-C(6) ) 1.79(6), P(1)-C(9) ) 1.80(6), C(6)-C(7) ) 1.41(8), C(7)-C(8) ) 1.40(8), C(8)-C(9) ) 1.42(8), Fe(1)-phopsholyl ) 1.644, Fe(1)-Cp ) 1.648; C(6)P(1)-C(9) ) 90.0(3), P(1)-C(6)-C(7) ) 112.2(5), C(6)-C(7)C(8) ) 112.6(5), C(7)-C(8)-C(9) ) 115.1(6), P(1)-C(9)-C(8) ) 110.0(5).
entry
L*
temp/°C
time/h
1 2 3 4 5 6 7 8e,f
3a 3b 12 13 13 14 15 16
20 20 20 20 0 20 20 0
120 72 72 72 72 72 72 160
yield/%b
